1. Field of the Invention
The present invention is concerned with an improvement in or relating to a composite type of vibration damping material (which may hereinafter be referred to as the vibration damper) comprising a metal sheet and a viscoelastic polymeric material and a soundproofing structure using such a material.
2. Statement of the Prior Art
As the composite type of vibration damping materials comprising a metal sheet and a viscoelastic polymeric material, there have been known one vibration damper formed of a thin steel sheet 1 and a rubber- or synthetic resin-base viscoelastic polymeric layer 2 bonded to both sides thereof, as illustrated in FIG. 22, and another vibration damper formed by sandwiching and bonding a viscoelastic polymeric layer 2 between two steel sheets 1, as illustrated in FIG. 23.
The vibration damper of FIG. 22 is called the non-constraint type and can damp vibration due to the "extensional deformation" of the viscoelastic layer in association with flexural vibration, whereas the vibration damper of FIG. 23 is referred to as the constraint type and can damp vibration due to the "shearing deformation" of the viscoelastic layer in association with flexural vibration. From a comparison of the non-constraint and constraint types of dampers, it is found that there is a considerable difference therebetween in that the constraint type is effective with a thin viscoelastic layer, but the non-constraint type needs a viscoelastic layer three to four times as thick as a steel sheet.
Said composite type of vibration dampers are used in a wide range of industrial fields such as automotive engine mounts, disc brakes, oil pans, transmissions, compressors, air cleaners, brake clutches, electronic ranges, speakers and players.
As mentioned above, the aforesaid composite type of vibration dampers are now increasingly used in various industrial fields due to their superior damping capacity. In actual applications, however, they are often used in severer environments where various factors such as temperature, pressure, oils, solvents and coolants are applied alone or in combination, rather than in relatively moderate environments at normal temperature, under no load and in the atmosphere. There is thus still demanded a vibration damping material that can sufficiently meet such requirements.
In particular, the aforesaid composite type of vibration damper is intimately related to the thickness of the metal sheet and viscoelastic layer forming it. One problem with this damper is that the thicker the metal sheet, the higher the mechanical strength but the lower the damping capacity. Another problem is that increases in the thickness of the viscoelastic layer result in decreases in both the resistance to heat and pressure and the mechanical strength. This vibration damper should thus be designed while taking such problems into account.